ConsecutiveAppendsShouldReuse rule

This pull refers to a new PMD rule to check consecutives appends on StringBuilder/StringBuffer objects. To improve the performance, it is recommended to do the consecutives calls over the result (same) object, instead of separated statements. For example:
```
String foo = " ";

StringBuffer buf = new StringBuffer();
buf.append("Hello"); // poor
buf.append(foo);
buf.append("World");

StringBuffer buf2 = new StringBuffer();
buf2.append("Hello").append(foo).append("World"); // good
```
I hope this helps.
Regards